Magneto-optic recording media are also known by several other names: thermomagnetic media, erasable optical media, beam addressable files, and photo-magnetic memories. All of these terms apply to a storage medium or memory element which responds to radiant energy permitting the use of such energy sources as laser beams for both recording and interrogation. Such media modify the character of an incident polarized light beam so that the modification can be detected by an electronic device such as a photodiode.
This modification is usually a manifestation of either the Faraday effect or the Kerr effect on polarized light. The Faraday effect is the rotation of the polarization plane of polarized light which passes through certain magnetized media. The Kerr effect is the rotation of the plane of polarization of a light beam when it is reflected at the surface of certain magnetized media.
When a magnetizable amorphous film is deposited on a reflector, the magneto optic rotation is increased because the Faraday effect is added to the Kerr effect. The former effect rotates the plane of polarization of the light as it passes back and forth though the magneto-optic layer while the Kerr effect rotates it at the surface of the layer.
A change in orientation of polarization of the light is caused by the magneto-optical properties of the material in the bit or site on which the polarized light is incident. Thus, the Kerr effect, Faraday effect or a combination of these two, is used to effect the change in the plane of light polarization. The plane of polarization of the transmitted or reflected light beam is rotated through the characteristic rotation angle .THETA.. For upward bit magnetization, it rotates .THETA. degrees and for downward magnetization -.THETA. degrees. The recorded data, usually in digital form represented by logic values of 1 or 0 depending on the direction of bit magnetization may be detected by reading the change in the intensity of light passing through or reflected from the individual bits, the intensity being responsive to the quantity of light which is rotated and the rotation angle.
The main parameters that characterize a magneto optic (MO) material are the angle of rotation, the coercive force (H.sub.c), the Curie temperature and the compensation point temperature. The medium is generally comprised of a single element or multicomponent system where at least one of the components is an amorphous metal composition. Binary and ternary compositions are particularly suitable for these amorphous metal alloys. Suitable examples would be rare earth-transition metal (RE-TM) compositions, such as Gadolinium-cobalt (Gd-Co), Gadolinium-iron (Gd-Fe), Terbium-iron (Tb-Fe), Dysprosium-iron (Dy-Fe), Gd-Tb-Fe, Tb-Dy-Fe, Tb-Fe-Co, Terbium-iron-chromium (Tb-Fe-Cr), Gd-Fe-Bi (Bismuth), Gd-Fe-Sn (Tin), Gd-Fe-Co, Gd-Co-Bi, and Gd-Dy-Fe.
The susceptibility of Re-Tm alloys to corrosion is well known. When used as a thin film in MO disks, the alloy is usually protected from contact with the ambient atmosphere by surrounding it with dielectric layers, partially as an effort to prevent corrosion by water which may be aided by other materials (e.g., chlorides). The RE-TM alloy and surrounding dielectric layers are often deposited on a transparent substrate, e.g. transparent polymer such as polycarbonate or polymethylmethacrylate. Such protection prevents observable corrosion for short periods (several months) of time under ambient conditions. However, extensive lifetimes (5-10 years) without corrosion are required of optical disks.
Because the active MO layer is usually quite thin, once significant corrosion has occurred the corrosion sites can become apparent as transparent or bright spots on the recording medium where the active RE-TM alloy may have been disrupted or converted to transparent oxide, exposing the underlying reflector. The result of this corrosion is a loss of stored information. Over time, the corrosion sites grow and increase the amount of lost information.
Previous literature references to corrosion of MO media indicate that additions of titanium, platinum and other elements are effective in inhibiting corrosion in samples of bare MO media exposed to aqueous salt solutions (Imamura, N., et al., "Magneto-Optical Recording on Amorphous Films", IEEE Transactions on Magnetics, September 1985, p. 1607 and Kobayashi, et al., IEEE Translation Journal on Magnetics in Japan, August, 1985). However, the present inventors have found that complete multilayer disk constructions do not exhibit the same corrosion performance as bare MO films.